FIG. 9 is a view showing a structure of a general high pressure discharge lamp such as an ultra high pressure mercury lamp. The high pressure discharge lamp 6 includes: a bulb 2 made of fused quartz; electrodes 7 disposed in a light emitting part 2a of the bulb 2 in a manner that the electrodes 7 face each other with an interval of 1.5 mm or less; molybdenum foils 4 disposed in sealing parts 2b of the bulb 2, respectively; and power supply leads 5 which are connected respectively to the molybdenum foils 4. The light emitting part 2a is filled with 0.15 mg/mm3 or more of mercury and with 10−5 μmol/mm3 to 10−2 μmol/mm3 of bromine.
FIGS. 10A and 10B are cross-sectional views each showing a structure of the electrode 7 in the high pressure discharge lamp of FIG. 9. The electrode 7 includes an electrode core bar 70 and a coil 75 covering the electrode core bar 70. In FIG. 10A, the leading end side of the electrode core bar 70 is covered with the coil 75, and the leading ends of the electrode core bar 70 and the coil 75 are melted to form a dome-shaped leading end portion. Meanwhile, in FIG. 10B, the electrode core bar 70 includes a small-diameter section 71 and a large-diameter section 72. The leading end side of the large-diameter section 72 is covered with the coil 75, and the leading ends of the large-diameter section 72 and the coil 75 are melted to form a dome-shaped leading end portion.
Generally, the electrode coil has a function of adjusting the temperature of the electrode, and thereby the discharge state, discharge characteristic, and the like are determined.
The temperature of the electrode becomes high and exceeds 2000 degrees during the driving of the lamp, and the coil 75 is also thermally affected. In the configuration as shown in FIG. 10, the coil 75 may spring-back in high temperature and expand toward the molybdenum foil 4 (rightward in FIG. 10), even if the coil 75 is wound densely right after being manufactured. As the coil 75 for adjusting the electrode temperature deforms in this manner with the elapse of driving period, the temperature condition of the electrode also changes. Thus, there occurs a problem that the discharge characteristic and the like vary among the individual electrodes.
As a measure against such problem of spring-back, Patent Document 1 discloses a configuration of integrating a coil and a small-diameter section (shaft) by melting. Specifically, as disclosed in FIG. 4 of the cited example, a coil is wound around a shaft (50) into a tapered shape (54), and the tapered portion is melted to form a leading end portion (20). In addition, as disclosed in FIG. 9 of the Document, a configuration is disclosed in which not only a leading end side (122) of the coil but also a terminal end side (124) thereof is melted to a shaft (126).